memory_hotplug: Optimize memory onlining

Optimize memory onlining by freeing up to 4MB at a time of blocks
instead of just page size. Calculates alignment of pages for
maximum grouping size.

Change-Id: I4e994cc50083980e9d36a1edd53d4ffe1f8478a6
Signed-off-by: Dylan Hatch <dhatch@codeaurora.org>